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Understanding 4:1:1 and 4:2:2 Sampling Rates

In the digital world video signals are encoded when recorded and decoded on playback. Depending on the digital format the video signal will be sampled at either 4:1:1 or 4:2:2 sampling.

The digital video signal has three components, they are the luminance (Y), a color value consisting of the luminance deducted from the color red (R-Y) and the color value of the luminance deducted from the color blue (B-Y). This is the same components that make up a Betacam SP and Digital Betacam signal. These three components, Y, R-Y and B-Y are also know as “YUV”.

During the digitizing process, the three parameters of the component video signal are assigned a numeric sampling value. Groups of four video pixels within each of the three components are looked at and samples are taken for recording. With a 4:2:2 sampled video signal, all four of the luminance pixels are sampled, two of the R-Y pixels are sampled and two B-Y pixels are sampled. This gives you a 4:2:2 sampling rate.

With a 4:1:1 signal, all four of the luminance pixels are sampled signal is sampled four times out of four, but only one pixel is sampled from each of the R-Y and B-Y. This lower sampling rate of the color components will result in less color information being recorded. This lower sampling rate affects the accuracy and intensity of the color in the video signal. Therefore, you may not want to use 4:1:1 when doing Chroma keying, graphics, and other compositing functions, since all of these require strong colors to be present in the video signal..

The advantage of 4:1:1 sampling is that you can record twice as much information onto the same area of video tape, thus providing twice as much recording/playback time within a given tape length. And, of course, the circuitry within the equipment is less expensive for a manufacturer to produce.

The 4:1:1 is the sampling rate is used with the consumer DV format, along with DVCAM and DVCPRO. The 4:2:2 sampling rate is used with Digital-S (from JVC), DVCPRO-50 (from Panasonic), Digital Betacam, D-1 and D-5.

Digital Signal Inputs and Outputs

All of the new digital video camcorders and digital VCRs have a variety of input and output connections available, including analog composite, analog Y/C (S-video), analog component (YUV), Firewire, and serial digital interface (SDI).

Typically on 4:1:1 (DV, DVCAM and DVCPRO) decks and camcorders, they will have composite, Y/C and on some units Firewire connections for input and output. This will give you several choices for transferring video between the different components in your editing system. In today's world of hybrid editing systems, you will find yourself using both analog and digital signals to go between your various pieces of equipment

You’ll find on most of the higher-end 4:2:2 digital editing decks they will have a serial digital interface (SDI) connections instead of Firewire. This will allow direct transfer the digital information from one machine to another to avoid the analog/digital encoding/decoding process. They will also have analog component (YUV) connections for transferring video to and from analog component VCRs, such as Betacam SP.

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